In processes referred to as catalytic cracking (ie English, Fluid Catalytic Cracking, or FCC) processes, it is known that a hydrocarbon charge is vaporized by bringing it into contact at a high temperature with an appropriate cracking catalyst which is maintained in suspension. Once a desired range of molecular weights has been achieved by cracking, with a corresponding lowering of the boiling point, the catalyst is rapidly separated from the lighter products obtained; the catalyst is subsequently regenerated by the combustion of coke deposited on its surface during the reaction, and then returned to the reaction zone together with the hydrocarbon charge.
In practice, the catalyst which has been regenerated (at a temperature which usually exceeds 600.degree. C.) and the charge to be treated are brought into contact continuously in a vertical or inclined tubular reactor. The latter, when working in ascending manner, is frequently referred to by specialists in the field by the term "riser", and is designated by the term "dropper" when it works in descending manner. The charge, usually preheated to a temperature of 80.degree.to 400.degree. C., is injected at a pressure of between 0,7.times.10 and 3,5.times.10 pa, and vaporizes and then cracks when coming into contact with the active sites of the catalyst, while effecting pneumatic conveyance of the grains of the catalyst, the desired average size of which is approximately 70 microns. After a contact time in the order of 0,1 to 10 seconds, the hydrocarbon vapours, at a temperature in the order of 475.degree.to 575.degree. C., are separated from the spent catalyst by means of a ballistic separator at the outlet of the tubular reactor. This separator is provided in a zone above which the hydrocarbon vapours rise, which vapours, after recovery of the catalyst fines therefrom by a cyclone, is dispatched to a fractionation apparatus. The grains of catalyst, under gravity, fall below said zone into a dense fluid-bed medium where, in order to be separated completely from the hydrocarbons still present in their pores, they are stripped by means of vapour. The stripped grains of catalyst are then removed to a regenerator where their catalytic activity is restored by combustion of coke deposited thereon during the cracking reaction.
During regeneration, combustion heat is distributed between the catalyst (approximately 70%) and fumes produced by the regeneration. The regenerated catalyst is recycled to the reaction zone, where the portion of the heat of combustion of the coke taken up by the catalyst in the regenerator is used to vaporize the charge, to provide reaction heat (the cracking reaction being endothermic) and to compensate for various thermal losses, thus ensuring the thermal equilibrium of the unit. The duration of an average cycle, for the catalyst, is approximately 15 minutes.
The FCC process is therefore employed such that the cracking unit is in thermal equilibrium, all the necessary heat being supplied by the combustion of the coke deposited on the grains of catalyst in the course of the cracking reaction. The relationship "preheating temperature of the charge/circulation of the regenerated catalyst" is therefore adjusted so as to obtain the desired reaction temperature in the entire reaction zone and, in particular, at the exit of the reactor.
The quantity of coke deposited on the catalyst is, therefore, a fundamental variable of the reaction, since it contributes to the supply of the heat required for the cracking of the hydrocarbons. The quantity of coke deposited on the grains of catalyst during the reaction is, however, frequently greater than that required to ensure the thermal equilibrium of the unit:
this is the case in particular when the hydrocarbon charges to be cracked are rich in heavy products such as asphaltenes or compounds having a high metal content;
this also frequently occurs because of a poor separation of the products of the cracking reaction or an inadequate stripping of hydrocarbons from the grains of catalyst coming from the cracking reaction.
This excess coke sent into the regenerator is due, at least partly, to the fact that not inconsiderable quantities of hydrocarbon residues (the hydrogen content of which may be between 5 and 10% by mass) cannot be separated from the grains of catalyst by the usual separation means. This results in too high a regeneration temperature, which is detrimental to the proper functioning of the reactor, and which is to the detriment of the quantity of product quality which can be obtained and which is recovered in the fractionation zone.
The most recent developments in the field of catalytic cracking have heretofore aspired to meet the afore-mentioned difficulties:
either by withdrawing excess heat from the regenerator so as to limit the increase in the regeneration temperature,
or by carrying out the regeneration in two stages which permits reaching far higher final regeneration temperatures for the catalyst.
The present invention aims to improve substantially the separation of the effluents of the cracking reaction and the stripping of the catalyst, so as to limit losses in hydrocarbon residues and to regenerate spent catalyst containing essentially only such coke as is necessary to ensure thermal equilibrium of the unit.
In fact, in the chambers for the separation of the effluents and the stripping of the spent catalyst used in the past, two distinct zones can be distinguished. In a first zone, or disengagement zone, a ballistic device of the kind known per se (see, for example, U.S. Pat. Nos. 2 420 558, 4 057 397, 4 478 708, or French Pat. Nos. 2 574 422 and 2 576 906) permits the downward movement of the grains of catalyst, while the hydrocarbon vapours rise upwards and are, after separation of the fines by means of a system of cyclones, dispatched to the fractionation zone. This operation which is most frequently carried out in a dilute fluidized phase, ensures a separation which is both rapid and effective between a substantial portion of hydrocarbon vapours and the grains of catalyst. In the second zone, taking place in a dense fluidized phase, below the ballistic-separation zone just mentioned, is a stripping operation during which the transport and the recovery of the gaseous hydrocarbons carried in the catalyst suspension are ensured by a counter-current stripping by means of a gaseous fluid such as water vapour. It is essential that the bringing into contact of catalyst and stripping fluid is effective and that any remixing is minimized. Generally, the stripping itself takes place in a dense phase, in a chamber usually characterized by an elevated ratio of height to diameter. This chamber is frequently provided with internal baffle-plates, in order to promote the contact of the catalyst in suspension with the stripping fluid.
In this second zone, the desorption of the heaviest hydrocarbons trapped on the catalyst is promoted by maintaining a partial pressure as low as possible of the hydrocarbons in vapour phase, relative to their so called bubble or blistering pressure, ie by maintaining an elevated temperature and low pressure. The use of very polar stripping fluids, such as water vapour, which are much more readily absorbed than the hydrocarbons, tends to promote the desorption of the hydrocarbons.
The stripping reaction, either by desorption or by transport of the hydrocarbons carried along, is relatively rapid. It is therefore useless to attempt to seek a greater stripping effectiveness by prolonging the time of catalyst contact with the stripping fluid because, during the stripping operation, the conditions are equally favourable to reactions for the coking of heavy hydrocarbons with the production of hydrogen and, more particularly, methane; the net result therefore is a reduction of the hydrogen in the residual coke remaining on the spent catalyst, to the benefit of the production of light gases.